Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2000-05-19
2002-03-12
Rosasco, S. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
C378S035000
Reexamination Certificate
active
06355385
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to microlithography as performed using a charged particle beam (e.g., electron beam or ion beam) as used in the manufacture of semiconductor integrated circuits, displays, and the like. More specifically, the invention pertains to reticle “blanks” as used to make pattern-defining reticles for use in such microlithography, and to methods for manufacturing such reticle blanks.
BACKGROUND OF THE INVENTION
The dramatic progressive reduction in the sizes of circuit elements in integrated circuits that has occurred in recent years has created a need for image resolution better than that obtainable using optical microlithography systems that are limited by the diffraction of light. This has led to the ongoing development of microlithography (projection-exposure) systems that, instead of using light, employ an X-ray beam or a charged particle beam such as electron beam or an ion beam.
Current charged-particle-beam (CPB) systems include electron-beam pattern-drawing (“direct-write”) systems in which an electron beam is used to form a pattern directly (i.e., without having to project a pattern onto the wafer). Because of the current ability to stop an electron beam down to a spot diameter of a few Ångstroms, high-resolution sub-micron patterns can be formed in this way. A major drawback of direct-write systems is the fact that the pattern is drawn element-by-element and line-by-line (i.e., by “direct writing”). To draw a finer element, the electron beam simply is stopped down further to a smaller spot diameter. However, reducing the spot diameter increases the amount of time (“writing time”) that must be expended to draw the entire pattern. Increasing the writing time correspondingly reduces throughput and increases device-production costs. Consequently, direct-write systems are impractical for mass production of chip-containing wafers.
The shortcomings of direct-write systems has motivated a large amount of development effort currently being directed to the development of a practical CPB microlithography system that projects (with demagnification) a pattern image from a “reticle” or “mask” to the wafer. Such systems are termed “reduced-image projection-exposure” CPB microlithography systems, in which a reticle defining the prescribed pattern is illuminated by a charged particle beam (e.g., electron beam), and a reduced (demagnified) image of the pattern located within the range of illumination is transferred onto the wafer by a projection lens.
By “demagnification” is meant that the image as formed on the wafer is smaller (usually by an integer factor such as ¼ or ⅕) than the corresponding illuminated region on the reticle.
As noted above, the pattern is defined on a “reticle” (sometimes termed a “mask,” but generally herein the term “reticle” is used). Two general types of reticles are known. A first type is termed a “scattering-membrane” reticle
21
, a portion of which is shown schematically in FIG.
7
(
a
). The scattering-membrane reticle comprises a reticle membrane
22
on which regions
24
are formed. The regions
24
are of a substance that scatters particles of a charged particle beam incident from above. The reticle membrane
22
is sufficiently thin to be transmissive to particles of the incident beam and thus exhibit essentially no scattering. The regions
24
, in combination with the transmissive membrane
22
, define the elements of the pattern. A second type of reticle is termed a “scattering-stencil” reticle
31
, a portion of which is shown schematically in FIG.
7
(
b
). The scattering-stencil reticle comprises a reticle membrane
32
(typically made of silicon) having a thickness (approximately 2 &mgr;m) sufficient to scatter particles of the incident beam. The membrane
32
defines through-holes
34
that are transmissive to particles of the incident beam. The through-holes
34
, in combination with the membrane
34
, define the elements of the pattern.
In CPB microlithography, it currently is impossible to project an entire pattern in one “shot.” As a result, the pattern as defined on the reticle is divided or “segmented” into multiple small portions termed “subfields”
22
a,
32
a
each defining a respective portion of the overall pattern and each containing a respective portion of the reticle membrane
22
,
32
. The subfields
22
a,
32
a
are separated from one another on the reticle by boundary zones (e.g., item
25
in FIG.
7
(
a
)) that do not define any portion of the pattern. Extending outwardly from the boundary zones
25
are support struts
23
that add substantial rigidity and strength to the reticle.
Each subfield
22
a,
32
a
represents an area of the reticle that can be exposed at any one instant, and each subfield is typically approximately 1-mm square in size. Hence, on the reticle, the entire pattern to be transferred to a chip-sized area (a “die” corresponding to a semiconductor chip) on the wafer is divided into a large number of, typically, 1-mm square subfields. The subfields are exposed individually. As the subfields are thus “transferred” to the wafer, the respective images of the subfields are “stitched” together contiguously to form the entire pattern in each die.
As shown in FIG.
7
(
c
), during pattern transfer, the subfields
22
a,
32
a
are scanned in a stepwise manner by the charged particle beam to transfer, to a “sensitized” substrate (“wafer”)
27
, the respective pattern portions defined by the subfields. By “sensitized” is meant that the substrate
27
is coated with a material (termed a “resist”) capable of being imprinted with the projected subfield images. FIG.
7
(
c
) clearly shows the “reduction” or “demagnification” of the images that occurs during projection, and the “stitching together” of the subfield images on the substrate in a contiguous manner.
Reticles for CPB microlithography normally are manufactured from “reticle blanks” that include the reticle membrane and the supporting struts. The lattice-like arrangement of the support struts defines intervening spaces on the reticle membrane in which the various subfields will be formed. A conventional process for making a reticle blank that includes a silicon membrane is shown in FIGS.
6
(
a
)-
6
(
f
).
In a first step (FIG.
6
(
a
)), a silicon substrate having a (
100
) surface orientation is prepared, and boron is diffused (e.g., by thermal diffusion or ion implantation) into one major surface of the substrate to form an “active” silicon layer
12
(FIG.
6
(
b
)). During a later etching step, the active silicon layer acts as an etch-stop layer. The active layer
12
is also destined to become the silicon membrane of a reticle formed from the blank, with the remainder of the silicon substrate
11
(i.e., everything except the active layer
12
) being regarded as the silicon support portion
11
a
(FIG.
6
(
b
)).
Next, a silicon nitride film
13
is formed over the entire outer surface of the silicon substrate
11
(FIG.
6
(
c
)). A wet-etching mask
15
is formed by etching a pattern of “windows” (openings)
14
in the silicon nitride formed on the lower (in the figure) major surface of the substrate (FIG.
6
(
d
)). Although only one window
14
is shown in the figure, normally a large number of such windows
14
are formed, corresponding to the number of subfields into which the reticle will be divided.
Next, the silicon substrate
11
(with etching mask
15
) is immersed in an etching solution such as potassium hydroxide (KOH). The KOH solution wet-etches the silicon support portion
11
a
in the windows
14
not protected by silicon nitride. The etching solution removes silicon mainly in the depthwise direction. The etching rate drops abruptly when etching has reached the active silicon layer
12
, thus stopping the depthwise etch (FIG.
6
(
e
)).
If the boron concentration in the active silicon layer
12
is less than 2×10
19
atoms/cm
3
, no significant drop in etch rate will occur, and etching will proceed through the active silicon layer
12
. Hence, to serve as an effective etch
Klarquist & Sparkman, LLP
Nikon Corporation
Rosasco S.
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